Laser-diode arrays are attractive for use in many applications, such as laser range finding (e.g., LiDAR, etc.), material processing, laser pumping, illumination, 3D imaging, gaming, and medicine, among others.
Typically, conventional laser-diode arrays are electrically driven in parallel, where all laser diodes are biased with the same voltage but each receives a different electric current (hereinafter referred to as simply “current”). Unfortunately, the aggregate current required for such an arrangement becomes impractical as the size of a laser-diode array increases—particularly for high-optical-power applications such as LiDAR. For example, an array of 20 laser diodes, each requiring 20 amps of current per device, requires an aggregate current of 400 Amps, while an array of 100 laser diodes that require 50 amps per device requires an aggregate current of 5000 Amps.
Such high-current requirements lead to significant complications at the system level. High-current power supplies and their associated cabling are prohibitively bulky and expensive for many applications. Miniaturization of power supplies and cabling is possible in some applications that require only short pulse, low duty-cycle operation (nano- to micro-seconds) because the total average power is low. Unfortunately, even for short-pulse power supplies, the ability to reach kA is still challenging.
As a result, laser-diode arrays have been developed such that their emitters are electrically connected in series, wherein a single voltage is applied across an entire string of serially connected laser diodes and the elements of the string share the same current. Typically, this is done by bonding individual emitter chips onto a common substrate and electrically connecting them via conventional wire bonding technology. Such an arrangement can dramatically reduce the current requirements for a laser-diode array while still delivering high current to each device.
Unfortunately, monolithic series-connected laser-diode arrays have proven to be difficult to fabricate in practice. As a result, electrically connecting the elements of a laser-diode array in series are normally based on conventional electrical approaches.
In some approaches, individual laser diodes are mounted onto an electrically insulating substrate on which a conductive pattern is defined. The bottom contact of each laser diode is electrically connected to a different electrically isolated region of the pattern. Conventional external electrical connections (e.g., wire bonds, tab bonds, ribbons, etc.) are then made between the top contact of each device and the region of the conductive pattern to which the bottom contact of the preceding laser diode in the string is electrically connected, thereby realizing a series-connected string of singulated laser diodes.
Another conventional approach for realizing a series-connected laser-diode string includes mounting a parallel laser array to the insulating substrate, followed by singulating the laser diodes (e.g., by sawing, laser cutting, etc.) to electrically isolate each laser diode from its neighbors. The electrically isolated laser diodes are then electrically connected in series using external electrical connections, as described above.
Such conventional approaches require complex packaging methods and external electrical connections, both of which increase size, cost and complexity of the series connected devices. Furthermore, external electrical connections, such as wire bonds and the like, introduce parasitic electrical issues that can limit short-pulse, high-current, high-frequency operation.
The need for a practical, low-cost, series-connected laser diode array suitable for a broad range of applications remains, as yet, unmet in the prior art.